1. Field of the Invention
The invention relates to an x-ray tube of the rotating-anode type in which rotation of the anode takes place by means of magnetic bearings.
2. Description of the Prior Art
High-power x-ray tubes usually make use of rotating anodes which are still very commonly designed for rotation by means of mechanical bearings. By reason of the high speed of rotation which can attain 20,000 revolutions per minute, these mechanical bearings are subject to rapid wear and are frequently the cause of withdrawal of x-ray tubes from service.
One significant improvement consists in mounting the rotating anode with magnetic bearings. These bearings usually comprise electromagnets which are mounted in opposition in pairs and a rotor which is rigidly fixed to the rotating anode in order to drive this latter in rotation, the rotor being maintained in equilibrium under the influence of the magnetic fields produced by the electromagnets. Thus the rotating anode and the mechanical parts which accompany this latter in rotational motion no longer have any mechanical contact with the remainder of the x-ray tube.
However, a new problem created by this improvement lies in the fact that the anode is isolated from the remainder of the x-ray tube, not only from a mechanical standpoint but also electrically. In consequence, both the connection of the anode to the high-voltage supply of the x-ray tube and the flow of anode current have been achieved only in an imperfect manner up to the present time. The solutions proposed are often sliding-contact or ball systems which are consequently mechanical, thereby losing part of the advantage of the magnetic-bearing system, namely the total absence of mechanical friction.
Another solution which permits the flow of anode current without any mechanical contact utilizes the emission of electrons generated by thermoemissive cathodes coupled mechanically to the rotating anode, these electrons being collected by fixed anodes. One of the main difficulties in this case consists in ensuring that, when these cathodes are set in rotation, they are supplied with the necessary energy for producing a rise in the cathode temperature level which is sufficient to satisfy the laws of thermoelectronic emission.
Up to the present time, this difficulty has been circumvented by means of relatively complex and costly solutions which consist for example in utilizing induction phenomena for transferring to these cathodes the energy which is necessary in order to heat them. One solution of this type is described in a French patent Application published under No. 2,494,496.
This French patent Application describes the arrangement of a rotating-anode x-ray tube provided with magnetic bearings and with means for passing the anode current without any contact. In this French patent Application, a rotating anode 3 rigidly fixed to a rotor 5 is mounted on magnetic bearings 9, 10. In order to discharge the anode current, the rear end of the rotor 5 is provided with two cathodes 13, 14 which rotate about a shaft 12 on which is mounted an anode 20. The cathodes 13, 14 are provided with windings 15, 16 which are adapted to cooperate with a plurality of excitation coils 17 during the movement of rotation of the anode 3. These excitation coils 17 are disposed at intervals around the periphery of the x-ray tube and induce the heating voltage for the cathodes 13, 14 within the rotating windings 15, 16.
The practical application of this solution is complicated and carries a heavy cost penalty. Furthermore, problems of reliability may arise from the high temperatures to which the rotating windings 15, 16 are exposed. In point of fact, these windings 15, 16 must behave as electric insulators which are capable of withstanding these temperatures and are not liable to have a degassing effect. This requirement presents a technological problem which has not yet been satisfactorily solved.